A summary of the legal rules affecting roommates.
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When two or more people simultaneously sign the same rental agreement or lease -- or enter into the same oral rental agreement -- they are co-tenants and share the same legal rights and responsibilities. But there's a special twist. One co-tenant's negative behavior -- not paying the rent, for example -- can affect everyone's tenancy.
Co-tenants may decide to split the rent equally or unequally, depending on their own personal wishes. However, such agreements don't have any impact on the landlord. Each co-tenant is independently liable to the landlord for all of the rent. Landlords often remind co-tenants of this obligation by inserting into the lease a chunk of legalese which says that the tenants are "jointly and severally" liable for paying rent and adhering to terms of the agreement. If one tenant can't pay a share of the rent in a particular month, or simply moves out, the other tenant(s) must still pay the full rent. Landlords often insist on receiving one rent check for the entire rent -- they don't want to be bothered with multiple checks from co-tenants, even if each co-tenant pays on time and the checks add up to the full rent. As long as you have been advised of this policy in the rental agreement or lease, it's legal for your landlord to impose it.
A landlord can, legally, hold all co-tenants responsible for the negative actions of just one, and terminate everyone's tenancy with the appropriate notice. For example, two co-tenants can be evicted if one of them seriously damages the property or otherwise violates the lease or rental agreement. In practice, however, landlords sometimes ignore the legal rule that all tenants are equally liable for lease violations, and don't penalize a blameless one. If the non-offending roommates pay the rent on time, do not damage the landlord's property and can differentiate themselves from the bad apple in the landlord's eyes, the landlord will probably want to keep them.
Roommates make lots of informal agreements about splitting rent, occupying bedrooms and sharing chores. Your landlord isn't bound by these agreements, and has no power to enforce them. For all sorts of reasons, roommate arrangements regularly go awry. If you have shared an apartment or house, you know about roommates who play the stereo too loud, never wash a dish, always pay their share of the rent late, have too many overnight guests, leave their gym clothes on the kitchen table or otherwise drive you nuts. If the situation gets bad enough, you'll likely end up arguing with your roommates about who should leave.
As a general rule, you can't terminate your roommate's tenancy by filing an eviction action. The exception involves rentals governed by the few rent control statutes, such as the one in San Francisco, that allow a landlord to designate a "master tenant" -- usually a long-term tenant who was there first -- to perform many of the functions of a landlord. Master tenants have the right to choose -- as well as to evict -- tenants. If your municipality is subject to rent control, find out whether the scheme includes a provision for a master tenant.
The more you can anticipate possible problems from the start, the better prepared you'll be to handle disputes that do arise. First, try to choose compatible housemates. Before you move in, sit down with your roommates and create your own agreement covering major issues, such as: